Optical glass fibers are generally coated with two superposed radiation-cured coatings, which together form a primary coating. The coating which is in direct contact with the glass is called the inner primary coating and the overlaying coating is called the outer primary coating.
The inner primary coating is usually a relatively soft coating providing environmental protection to the glass fiber and resistance, inter alia, to the well-known phenomenon of microbending. Microbending in the coated fiber can lead to attenuation of the signal transmission capability of the coated fiber and is therefore undesirable. The outer primary coating, which is on the exposed surface of the coated fiber, is typically a relatively harder coating designed to provide a desired resistance to physical handling forces, such as those encountered when the fiber is cabled.
In telecommunications applications of optical fibers, multiple individual strands of coated fiber are usually packaged into larger structures, such as ribbon assemblies to maximize efficiency. However, after ribboning of fiber, the individual strands of fiber must be readily distinguishable from each other to provide individual identification during, for example, installation and repair. Color coding can be used to distinguish and identify individual fibers in a complex cable. Although several methods can be used to color code fiber, color coding can be done advantageously using a thin ink layer, such as from about 3 to about 10 microns, which is placed over the coated fiber before ribboning and cabling. Alternatively, a colorant can be incorporated into the outer primary coating to provide a colored outer primary coating.
Tape-like ribbon assemblies are prepared by embedding a plurality of individual color coded fibers in a supporting matrix material which, like the inner and outer primary coatings, is also radiation-curable to maximize production speed. Such ribbon assemblies usually contain from about 4 to about 12 color coded fibers. Cure of the matrix material occurs during the ribboning stage after the fibers have been color coded by ink. Hence, in a ribbon design, the ink layer resides between the ribbon's matrix material and the fibers' outer primary coating. This means that the ink layer's interfacial characteristics (e.g., surface energy, adhesion) must be carefully controlled to function properly with both matrix material and outer primary coating in the ribbon structure. If a colored outer primary coating is utilized in place of an ink coating, the colored outer primary layer's interfacial characteristics (e.g., surface energy, adhesion) must be carefully controlled to function properly with the matrix material and inner primary coating in the ribbon structure. In particular, the ability of a cured matrix material to be suitably stripped off the ink layer or colored outer primary coating, referred to as "break-out, is an important technical consideration. Break-out is generally carried out by a mechanical force, although chemical softening of the matrix with use of solvents is also known.
The term "ribbon assembly" as used herein includes the tape-like ribbon assembly described above, as well as optical glass fiber bundles. Optical glass fiber bundles can be, for example, a substantially circular array having at least one central fiber surrounded by a plurality of further optical glass fibers. Alternatively, the bundle may have other appropriate cross-sectional shapes such as square, trapezoid, etc.
Optical fiber color coding can be based on up to 12 or more colors. Although optical fiber inks were originally solvent-based or thermosetting inks, in more recent times, radiation-curable inks have been used to increase the speed of the inking process. In these ink compositions, pigment is dispersed in a radiation-curable carrier or base composition. In addition, ink compositions should not contain ingredients that can migrate to the surface of the optical glass fiber and cause corrosion. The ink composition should also not contain ingredients which can cause instability in the protective coatings or matrix material. Ink coatings for optical glass fibers should be color fast for decades, not cause attenuation of the signal transmission, be impervious to cabling gels and chemicals, and allow sufficient light penetration for fiber core alignment.
From the above, it is clear that optical glass fiber technology places many unique demands on radiation-curable ink compositions which more conventional technologies, such as printing inks, do not.
U.S. Pat. No. 4,900,126 (Jackson) discloses an optical glass fiber ribbon unit in which each of the individually coated optical glass fibers has a colored outer layer. Each of the optical glass fibers is further coated with a release agent which has a low affinity for the bonding material or the colorant material. An example of the release agent is teflon. The release agent creates a weak boundary layer at the interface of the colorant material and the matrix material whereby the matrix can be separated from the optical glass fibers without removing the colored layer on the individual optical glass fibers.
Published Japanese Patent Application No. H1-152405 discloses a radiation-curable ink composition containing an organic polysiloxane compound. The polysiloxane compound provides the ink coating with the ability to separate more easily from the matrix material in a ribbon assembly.
Published Japanese Patent Application No. 64-22976 discloses radiation-curable ink compositions containing specific radiation-curable oligomers. The ink composition provides an ink coating having adhesion to the outer primary coating which is separable from the matrix material in a ribbon assembly.
Published Patent application EP-A-614099 describes the use of a release agent such as a silicon oil or a fluororesin between the bundling layer and the coloring layer. When substantial amounts of silicone resins are used, incompatibility in the liquid and resultant imperfections in the cured matrix composition may result, which can lead to undesirable attenuation of signal light transmission.
There is a need for a ribbon assembly which is capable of providing break-out of the individual coated optical glass fibers, without requiring the use of a release agent.